Aerosols are a suspension of solid or liquid particles in a gas. Aerosols that include particles with diameters of 1 micrometer or larger can be characterized as coarse-particle aerosols. The particles in coarse-particle aerosols can be composed of mechanically produced solid particles, mechanically produced spray droplets, or atmospheric cloud droplets.
To most effectively study the physical and chemical characteristics of the aerosol particles, it is necessary to separate the particles from the aerosol gas and transport the separated particles to a point where they can be either collected for later study or scrutinized in situ with suitable analytical instruments. Often it is desirable to sample and study only a particle size or range of sizes of particles. For accurate sampling and analysis, it is also important that the particles to be sampled are effectively separated, i.e., isolated from other aerosol particles having sizes outside of the desired range. Furthermore, the separated particles must be transported to the region where they are collected or analyzed without colliding with the structure of the sampling device. Such collisions result in losses when the particles adhere to the structure. Even if the particles collide but bounce off the structure they may be contaminated through contact with the structure.
In the past, devices known as impactors have been used for separating particles from the aerosol gas. Generally, impactors consist of an impaction plate and means for causing the aerosol to flow toward the plate. The direction of the aerosol flow is abruptly changed near the impaction plate so that the particles with sufficient inertia will deviate from the flow where it changes direction and impact upon the plate. The particles that collect on the plate are subsequently collected and/or analyzed.
The determination of which particles will deviate from the flow and impact upon the plate depends essentially upon two parameters. These parameters are: the particle's inertia, as quantified by a parameter known as the "stop distance" L; and the minimum radius of curvature r.sub.c of the streamlines of the aerosol gas flow in the region where the flow changes direction. The streamlines are lines in the gas flow of the aerosol which are everywhere parallel to the direction of flow at a given instant. For spherical particles having a diameter of 1 micrometer or larger, the stop distance L is defined as follows: EQU L=V.sub.i mBf
where
V.sub.i =the impaction velocity, defined as the velocity of the particle at a distance upstream from the directional change of the aerosol flow. PA1 m=the mass of the particle. PA1 B=the mobility of the particle, defined as the particle velocity per unit of drag force under steady state conditions. In equation form: EQU B=1/(6.pi..eta.r.sub.p) PA1 n=viscosity of the aerosol gas. PA1 r.sub.p =particle radius. PA1 f=a correction factor for particles with upstream Reynolds numbers, Re.sub.i, greater than unity, in equation form: PA1 f=3.epsilon..sup.-3/2 Re.sub.i.sup.-1 [Re.sub.i.sup.1/3 .epsilon..sup.1/2 +arctan(Re.sub.i.sup.-1/3 .epsilon..sup.-1/2)-.pi./2. PA1 Re.sub.i =2r.sub.p .rho.V.sub.i /.eta.. PA1 .rho.=density of the gas. PA1 .epsilon..congruent.1/6, a numerical constant.
where
and
where
It is known that when the ratio of the stop distance L to the radius of curvature r.sub.c is much greater than unity for a particle, that particle will deviate from the streamlines of the gas flow and impact upon an object if one is present. The ratio of the stop distance L to the radius of curvature r.sub.c of the gas flow streamline is referred to as the Stokes number, Stk. Therefore, in order to separate particles of a known stop distance from a passing flow of the aerosol, the structure must be arranged so that the radius of curvature r.sub.c of the streamlines of the aerosol flow is small enough near the impaction plate to create a Stokes number of greater than 1 for that particular particle.
One problem with conventional impactors is that the separated particles have a tendency to bounce off the impaction plate and become reentrained within the flow. Furthermore, conventional impactors are designed to accumulate particles on the impaction plate prior to analysis. For some particles, such as cloud droplets, this accumulation makes accurate analysis of certain characteristics of these particles extremely difficult. More particularly, cloud droplets are typically comprised of a volatile solvent such as water containing various solutes such as sulfuric acid, nitric acid, bisulfate compounds and others. Once the cloud droplets are separated and impact upon the impaction plate, the liquid solvent undergoes mixing and, usually, uncontrolled evaporation. Furthermore, the properties of the solutes (some of which, such as nitric acid, are volatile themselves) undergo rapid changes, thereby making it practically impossible to capture and measure these solutes with conventional impactors. Nonvolatile solutes may also undergo physical or chemical changes due to contamination through contact with the impaction plate and/or other stolutes accumulated on that plate.
Some impaction devices, known as virtual impactors, are configured to avoid the problem of particles bouncing off the impaction plate and becoming reentrained in the aerosol flow. These devices are known as virtual impactors because there is no particle impaction with a solid object at the point where particles separate from the flow. Rather, the separated particles are directed from the point of separation to a collection region located away from the remainder of the aerosol flow. For example, in one type of virtual impactor such as that shown in U.S. Pat. No. 4,301,002, issued to Loo, the aerosol flow is directed from a nozzle toward a spaced-apart hollow collection probe. Suction is applied across the space between the nozzle and the probe in order to divert a portion of the aerosol flow away from the probe. The diverted portion of the flow carries most of the small-inertia particles from the probe. The remaining undiverted portion of the original aerosol flow (hereinafter "sampling flow") is directed into the collection probe. The larger particles (the precise size of which depends upon the magnitude of the applied suction) separate from the diverted flow and are carried by the sampling flow into the collection probe where they are collected on a filter located away from the flow diversion region.
Past virtual impactor designs may reduce the particle reentraining problem discussed with respect to conventional impactors, but since the sampling flow is merely an undiverted portion of the original aerosol flow, the collected particles will necessarily include some small-inertia particles in the same concentration as in the general aerosol. As noted, unseparated small-inertia particles will adversely affect the accuracy of coarse particle analysis. Past virtual impactors also lack provisions for minimizing losses or contamination due to particle collisions with the walls of the probe or conduit in which the separated particles flow. Furthermore, these devices propose no effective means of transporting and treating separated liquid droplets so that the solvent and solute of that droplet can be accurately analyzed in situ.